The present invention concerns optical, i.e., light-conducting fibers, which are transparent to infrared radiation and can be used for transmission of energy, e.g., from high energy infrared lasers (e.g., CO and CO.sub.2 -lasers).
The problem of overloading of the existing limited capacity energy-flow-carrying conductor lines, which is increasingly becoming a problem even now, has given rise to a general search for new energy-transmitting conductive materials. This research has most recently been extended to the field of optical energy transmission which has acquired increasing significance as an alternative to conventional electrical energy transmission systems. High energy light sources, such as lasers and light-emitting diodes (LED) are currently being used in medical technology, materials processing and telecommunications. However, so far, this type of transmission has been confined to non-flexible beams or transmission cables comprising copper. Inorganic glass, which is cheap by comparison with copper, is currently being developed into a genuine substitute material for use in telecommunications transmission systems. This new transmission system consists of a source--the transmitter, which typically is either a laser or an LED, to which the transmission line, i.e., the glass fiber, is coupled, and a receiver joined to the end of the line. This basic assembly can be repeated several times in succession. The quality of the transmission depends in all practical applications on the amount of transmittable energy, i.e., on the transmittancy of the line.
Particularly favorable transmission capacities, i.e., high information densities, can be achieved in fibers of this kind with the aid of refractive-index-gradient profiles in the light conducting optical fiber core. In refractive-index gradient fibers the light is transmitted, not by total reflection, but by re-direction into the interior of the fiber core due to the given refraction-gradient profile.
With special reference to telecommunications in the infrared region of the spectrum, the work of Kao & Hockhaus (Proc. IEEE 113 (1966) 1151) clearly indicates the potential, as well as the aims and targets, for the development of suitable communication fibers. Even in this early work, it was recognized quite clearly that there are two vital factors which must be given equal consideration, namely, transmission losses (e.g., due to absorption or other factors which attenuate the strength of the energy carrying the information over the length of the fiber), and transmitting capacity (i.e., the quantity of information which can be transmitted). Initially, research emphasis was centered on reducing the losses due to absorption in the wavelength range of the light sources which would ultimately be used. Owing to the great difficulties which were encountered in earlier attempts to make low-loss light conductors, research was concentrated for a very long time on stepped-index fibers while the equally important demand for satisfactorily high transmitting capacity was approached rather late.
Initially the light sources are likely to be lightemitting diodes (LED). These have an incoherent and weakly directed emission in the wavelength range of 750 to 900 nm. Later generations will probably use semiconductor injection lasers, i.e., coherent light sources in the same wavelength range. The wave band for the laser is determined, for example, by suitable choice of a particular AlGaAs laser as the semiconductor laser. The shift toward the infrared region of the spectrum will progress to increasingly longer wavelengths in the future. Consequently, the infrared region is of particular interest.
In addition to pure absorption losses and those due to some light scattering or diffusion, other additive causes of light loss exists such as losses due to bending of the conductors, to geometrical variations in the fiber cross section etc. For material processing and medical technology, flexible transmission lines are needed with a minimum of such losses even over longer transmission distances. By virtue of their high transmittancy, they must provide an optimum of energy transmission from infrared lasers which have outputs in spectral regions above 1 .mu.m and provide highly homogeneous impulses.
In order to obtain the required low-loss quality in the optical fiber it is essential to achieve extremely low absorption, i.e, to produce fibers containing an absolute minimum of absorbing impurities. Such fibers, insofar as these exist at all today, are almost exclusively produced by the CVD technique, i.e., by precipitation from the vapor phase. This CVD technique is quite old and dates as far back as approximately 1940 (J.O.S.A. 36 (1946) 702 ff). It is based on the pyrolytic oxidation of metallic chlorides. These metallic chlorides are often present in the liquid state, e.g., as SiCl.sub.4 or GeCl.sub.4, or can be easily liquified under pressure, e.g., BCl.sub.3. The advantages of such liquid metallic chlorides, or metallic halides, derive from their ready distillability so that it is possible to satisfy the requirement of a low absorption quality in the fiber by virtue of the purity obtainable with such raw materials. This same vapor deposition or precipitation process is also very widely used in semiconductor technology.
Currently there are two methods which are primarily used for making light-conducting glass fibers for optical fiber conductors in telecommunications. Although there are essential differences between these two methods, they both use the CVD process to produce an oxide precipitate for the optical fiber. The first patents in the field of glass fiber production for optical fiber conductors in telecommunications apply this process to obtain a white, soot-like precipitate which, according to experience borrowed from semiconductor technology, can be obtained with a remarkable degree of purity. Later patent applications have reverted to the older method of producing a glassy material directly from the gaseous phase by application of the CVD process. Both of these methods are used for applying either an exterior layer coating of a material having a lower refractive index to an extremely clean and pure rod of flint glass, or an interior layer coating of higher refractive index material to a tube of flint glass. The rod-like preforms which are produced in this way are then drawn out into fibers. The desired refractive index gradient is produced during the manufacture of the preform by varying the composition of the deposited precipitates (coatings). For the external layer coating process, the refractive index of the material is reduced stepwise with increasing distance from the axis of the preform. For the internal layer coating method, on the other hand, the refractive index is stepwise increased with increasing approach to the fiber preform axis. Following the interior layer coating process, when all required layers have been deposited, the internally-coated tube is collapsed to form a rod-like preform.
The actual precipitate, whether it be vitreous or sooty, is obtained by propelling the metallic halide molecules by and in a stream of oxygen into a temperature controlled region where they react with the oxygen and are precipitated as oxides. The halides evaporate. Depending on the kind of metals used in the process (Si.fwdarw.SiO.sub.2 ; Ge.fwdarw.GeO.sub.2 ; Ti.fwdarw. TiO.sub.2,B.fwdarw.B.sub.2 O.sub.3 etc.), the required reaction temperature is generated by oxyhydrogen burners or plasma. In these cases the pyrolytical reaction is an oxidation by the additionally supplied propellant gas, i.e., oxygen.